Put the potatoes in a large pot and cover them with cold water. Bring the water to a boil, then simmer until the potatoes are fork-tender, about 30 minutes from the cold start.

Set a colander over a large bowl and drain the potatoes, saving all the cooking water. Return the potatoes to the pot.

Add the butter, sour cream, cream cheese, salt, garlic powder, onion powder, and black pepper. Mash with a potato masher, adding the reserved cooking water ½ cup at a time, until the potatoes are smooth. If you use a stick blender instead, blend in short pulses and stop as soon as they're smooth.

Fold in 1 cup of the cheddar, 6 slices of the crumbled bacon, and 3 tablespoons of the green onions. Taste and add more salt if needed.

Spread the potato mixture evenly in the baking dish. Sprinkle the remaining 1 cup cheddar over the top, then the remaining bacon and the remaining 1 tablespoon green onions.

Set the dish on a rimmed baking sheet and bake uncovered for 25 to 30 minutes, until the cheese is melted and bubbling and the casserole has puffed up in the dish.

Optional: move an oven rack about 6 inches below the broiler and broil for 2 to 3 minutes, until the cheese browns in spots. Watch closely so the bacon and green onions don't burn.

Let the casserole rest for 5 to 10 minutes so it settles and holds together when scooped. Sprinkle with parsley, if using, and serve.